Cardiovascular disease increases markedly with advancing age in our Veteran population. In aging, myocardial injury is increased during ischemia (ISC)-reperfusion (REP) and leads to heart failure. Mitochondrial-driven injury during ISC-REP in aged hearts is mediated by the electron transport chain (ETC) due to increased production of reactive oxygen species (ROS) and damage to the mitochondrial phospholipid cardiolipin (CL). Most therapeutic strategies that effectively decrease cardiac injury in younger hearts fail in aged hearts. In contrast, we found that a transient, reversible inhibition of ETC complex I during early REP decreased injury in the aged heart. The key mechanism and target of protection at the cellular level that mediates ETC-driven protection remains a critical unanswered question. In addition to mitochondria (MITO) damage, endoplasmic reticulum (ER) stress contributes to cardiac injury. MITO and ER are juxtaposed through shared structures of mitochondria-associated membranes (MAM). MAM contribute a critical role in calcium regulation and lipid metabolism between MITO and ER. Disruption of MAM integrity likely mediates pathologic ?cross talk? between ER and MITO and increases cardiac injury. We found that CL exists in MAM and the ER with a composition distinct from MITO. Our previous work showed that CL content in MITO is decreased and exhibits an increase in oxidized CL content following ISC-REP in the aged heart. ER is a key site for the remodeling/repair of damaged CL. We hypothesize that CL is a key mediator of the ?cross talk? between MITO and ER via the MAM, and that trafficking of damaged CL to ER via the MAM is a mediator of MAM disruption in the aged heart during ISC-REP. Age-induced ETC defects increase ROS production. MITO ROS can damage MAM and activate ER stress responses. We found that aging augments cardiac ER stress. MAM are uniquely positioned to sustain damage from both MITO and ER. The increased ROS from aged heart MITO may damage CL in the MAM and activate ER stress. We will evaluate the role of primary MITO dysfunction at baseline and then following ISC and early REP to MAM damage and CL modification in Aim 1. The role of ETC inhibition to protect MAM during ISC-REP is studied (Aim 1). The response of MAM and ER to ISC-REP in the aged heart is unknown (Aim 2). Acute, transient complex I blockade with a reversible inhibitor decreases injury in aged hearts. We found that metformin inhibits complex I in a dose-dependent manner, especially in ISC-damaged heart MITO. Next, we showed that metformin treatment only at REP in doses that inhibit complex I decreases infarct size in vivo following 24 hrs. REP. We hypothesize that modulation of complex I at REP in aged hearts will attenuate damage to the MAM with a subsequent decrease in ER stress that, in turn, minimizes the generation of dysfunctional MITO and cardiac injury. The partial inhibition of complex I is studied as a new upstream therapeutic intervention to attenuate cardiac injury and minimize post-infarction heart failure development in aged hearts (Aim 3).